This invention relates to a device to improve the images obtained using magnetic resonance angiography (“MRA”); and more particularly, in one aspect, to a device (as well as a method of using the device) that selectively compresses veins relative to arteries to control, time, delay and/or prevent excessive early venous enhancement relative to arterial enhancement and thereby improve and/or enhance MRA images.
Moving table MRA, sometimes known as bolus chase MRA, has had a revolutionary affect on imaging of the peripheral vasculature. Moving table MRA allows rapid imaging from the aorta to the feet, using one injection of a magnetic resonance (“MR”) contrast agent (for example, gadolinium), often within a few minutes. In operation, the patient is advanced, between discrete imaging stations, through the MR scanner as the bolus of MR contrast agent flows down the torso and legs. In this way, one bolus of MR contrast agent is “shared” among multiple imaging stations covering the peripheral arteries or it can be extended to cover the arteries of the entire body. Moving table MRA is described in U.S. Pat. No. 5,924,987, which is incorporated by reference herein.
However, in many patients, the MR contrast agent bolus may “travel” down the legs at a rate that makes it difficult for the MR scanner to adequately, fully and/or optimally image the patient at and move between the plurality of imaging stations. In this instance, sometimes referred to as fast flow, the bolus is not optimally “shared” among the imaging stations and enhancement of veins may occur or result in the more distal stations thereby interfering with visualization of the arteries in the subsequent images.
Such venous enhancement may be referred to as venous contamination and may occur in up to 50% of patients. Notably, it is particularly common in diabetic patients and patients who have cellulitis or foot ulcers. These are the very patients in whom obtaining high resolution peripheral MR angiograms may be critical.
There are many conventional techniques to suppress venous contamination. (See, for example, (1) Lee et al., “Distal lower extremity arteries: evaluation with two-dimensional MR digital subtraction angiography”, Radiology. 1998 May, 207(2):505-12; (2) Maki et al., “Utilizing SENSE to achieve lower station sub-millimeter isotropic resolution and minimal venous enhancement in peripheral MR angiography”, JMRI, April 2002, 15(4):484-91; (3) Ho et al., “High-spatial-resolution multistation MR imaging of lower-extremity peripheral vasculature with segmented volume acquisition: feasibility study”, Radiology June 2001, 219(3):835-41; (4) Leiner et al., “Three-dimensional contrast-enhanced moving-bed infusion-tracking (MoBi-track) peripheral MR angiography with flexible choice of imaging parameters for each field of view” JMRI, April 2000, 11 (4):368-77; (5) Wentz et al., “High-resolution magnetic resonance angiography of hands with timed arterial compression (tac-MRA)” Lancet 2003, 36149-50; (6) Vogt et al., “Venous compression at high-spatial resolution-resolution three-dimensional MR angiography of peripheral arteries”, Radiology 2004, 233:913-20; (7) Herborn et al., “Peripheral vasculature: whole body MR angiography with midfemoral venous compression—initial experience”, Radiology 2004, 230:872-8; (8) Bilicen et al., “Infragenual calf-compression for reducing venous contamination in contrast-enhanced MR angiography of the calf”, JMRI August, 2004 20(2):347-51, (9) Bilicen et al., “Optimized assessment of hand vascularization on contrast-enhanced MR angiography with a subsystolic continuous compression technique” AJR, 2004;182:180-2; and (10) Zhang et al., “Decreased venous contamination on 3D gadolinium-enhanced bolus chase peripheral MR angiography using thigh compression”, AJR 2004;183:1041-7).
Certain conventional techniques to suppress venous contamination employ faster imaging and/or faster table movement. An inherent problem with imaging faster to keep up with the bolus is that imaging faster reduces the time available for data acquisition. The result is that the final MR angiograms or images tend to have lower quality with lower resolution and less signal-to-noise (“SNR”). In addition, it may take time for the arteries distal to occlusive disease to fill in with the MR contrast agent to be adequately visualized. Notably, it is generally better to image longer for better filling of slowly filling arteries, higher resolution and greater SNR.
Drs. Meaney and Prince, in U.S. Pat. No. 5,924,967 (incorporated herein by reference), describe a number of techniques to suppress venous contamination. One such technique employs tourniquets to “slow down” blood flow and suppress venous enhancement. In this regard, the tourniquet is placed tight enough to compress veins, but not too tight because compressing arteries is undesirable. In general, compressed arteries may provide a false appearance of stenosis or occlusion if it is within the field of view of the MRA image. Notably, compressing an artery may be acceptable particularly if it is outside the field of view or already known to be free of significant disease.
Generally, tourniquets slow blood flow in and near the location around which the tourniquet is applied. In addition, where a tourniquet is applied to compress only veins, the more peripheral veins may become distended with blood. Thus, by placing or applying the tourniquet prior to injecting the MR contrast agent, the blood distending the veins is likely to be free of MR contrast agent. Once the MR contrast agent is injected, it flows down the arteries at high concentration, but then is rapidly diluted upon entering the dilated veins. This effect may reduce venous enhancement. It is especially helpful for the calf which commonly has venous enhancement if the gadolinium flows down the legs faster than the MR scanner table is advanced. Compression of veins also causes blood to back up and thereby slows down the arterial flow as well. Generally, the amount of compression obtained from typical elastic tourniquets is difficult to control and reproduce, which has a tendency to limit widespread applicability of the technique.
There is a need for, among other things, a thigh compression device and technique which may apply even or uniform (or substantially even or uniform) pressure “high” on the thigh. In addition, there is a need for a thigh compression device that fits and/or conforms to, the cone-like shape of the thigh, in order to limit, reduce and/or eliminate detrimental movement of the device. Moreover, there is a need for a thigh compression device and technique that (i) limits, reduces and/or eliminates the tendency to move, create motion or misregistration artifact, (ii) is relatively easy to operate or implement from outside the MR scanner and/or (iii) limits, reduces and/or eliminates “kinking” or “entanglement” of tubing (for example, via the MR scanner or other medical equipment) that is used to operate or implement the device. There is also a need for a thigh compression device that is easily and rapidly inflated and stays inflated in spite of leaks.